The present invention relates to change-speed gearing units for automobiles, and more particularly to a device for preventing metallic alien particles being trapped between a pair of change-speed gears of high meshing rate in the gearing unit.
In order to reduce meshing noises within a change-speed gearing unit, it is effective to increase the meshing rate of the change-speed gears. In one of the two usual manners for increasing the meshing rate, it is required to make each module of the gears as small as possible, and in the other usual manner it is required to make the whole depth of each tooth on the gears as great as possible. As a result of these requirements, each tooth tip on the gears is inevitably tapered, and the face of each tooth tip becomes narrow, which results in a reduction in the strength of each tooth tip on the gears. In the case that metallic alien matter such as worn-off metallic particles, metallic residue from the grinding of the shaft and gears, etc is trapped between the teeth of the gears, there will occur some damage to the teeth tips on the gears. On the other hand, it is very difficult to completely eliminate the metallic alien matter so as to avoid such damage in the actual use of the gearing unit.